Prior art CMOS devices manufactured with prior art semiconductor processes typically have polysilicon gate structures. Polysilicon, however, can be susceptible to depletion effects, which can add to the overall gate dielectric thickness in the CMOS device. Furthermore, as the effective physical gate dielectric thickness decreases, the polysilicon depletion contributes proportionally to the total dielectric thickness. It is, therefore, desirable to eliminate polysilicon depletion in order to scale gate oxide thickness.
Metal gates, on the other hand, are not as susceptible to depletion as polysilicon and are in many ways preferable to polysilicon for forming gate structures. Typical prior art semiconductor processes, however, do not incorporate n-type and p-type metal gates within the same device or integrated circuit. This is due, in part, to the complexity and cost of developing a semiconductor process that can reliably deposit metal gate structures of differing types into the same semiconductor device or integrated circuit.